Electric-motor driven parking brakes in motor vehicles are intended to replace the mechanical handbrake lever, actuated by muscle power. For this purpose, the electric-motor driven parking brake must be capable of building up the required brake actuation force in the brake cable within a very short time using the current available from the vehicle electrical system, while having the most compact dimensions possible. In addition, suitable measures must be provided to ensure that, after the brake has been pulled on and the motor switched off, the brake cannot be released as a result of an unintended backward movement of the brake cable.
Known electric-motor driven parking brakes have an electric motor which drives a spindle gearbox. The spindle gearbox converts the rotary movement of the electric motor into a translational movement of the spindle. The translational movement of the spindle is transmitted to a brake-actuating linkage, which is permanently connected to the brake cable. To avoid any unintended backward movement of the brake cable after the motor is switched off, the spindle gearbox is designed to be self-locking. This self-locking property is achieved by the thread on the spindle having a low pitch. However, a consequence of this is that the parking brake has a low efficiency because such a spindle drive has high frictional losses and, depending on its arrangement, a gearbox which is subject to further losses must generally be inserted between the motor and the spindle. Furthermore, such parking brakes have a large unit volume, because a larger motor is required due to the low efficiency. A further disadvantage consists in the fact that the large motor means that the current required cannot always be supplied by the vehicle's electrical system.